Hi Jens, Please queue this patchset up for the 5.20 merge window. UBLK_IO_NEED_GET_DATA is an important feature designed for Ming Lei's ublk_drv. All the patches have been reviewed by Ming and all test cases in his ublksrv[1] have been passed. [1] https://github.com/ming1/ubdsrv On 2022/7/28 20:39, ZiyangZhang wrote: > 1. Introduction: > UBLK_IO_NEED_GET_DATA is a new ublk IO command. It is designed for a user > application who wants to allocate IO buffer and set IO buffer address > only after it receives an IO request from ublksrv. This is a reasonable > scenario because these users may use a RPC framework as one IO backend > to handle IO requests passed from ublksrv. And a RPC framework may > allocate its own buffer(or memory pool). > > This new feature (UBLK_F_NEED_GET_DATA) is optional for ublk users. > Related userspace code has been added in ublksrv[1] as one pull request. > > We have add some test cases in ublksrv and all of them pass. The > performance result shows that this new feature does bring additional > latency because one IO is issued back to ublk_drv once again to copy data > from bio vectors to user-provided data buffer. > > 2. Background: > For now, ublk requires the user to set IO buffer address in advance(with > last UBLK_IO_COMMIT_AND_FETCH_REQ command)so the user has to > pre-allocate IO buffer. > > For READ requests, this work flow looks good because the data copy > happens after user application gets a cqe and the kernel copies data. > So user application can allocate IO buffer, copy data to be read into > it, and issues a sqe with the newly allocated IO buffer. > > However, for WRITE requests, this work flow looks weird because > the data copy happens in a task_work before the user application gets one > cqe. So it is inconvenient for users who allocates(or fetch from a > memory pool)buffer after it gets one request(and know the actual data > size). For these users, they have to memcpy from ublksrv's pre-allocated > buffer to their internal buffer(such as RPC buffer). We think this > additional memcpy could be a bottleneck and it is avoidable. > > 2. Design: > Consider add a new feature flag: UBLK_F_NEED_GET_DATA. > > If user sets this new flag(through libublksrv) and pass it to kernel > driver, ublk kernel driver should returns a cqe with > UBLK_IO_RES_NEED_GET_DATA after a new blk-mq WRITE request comes. > > A user application now can allocate data buffer for writing and pass its > address in UBLK_IO_NEED_GET_DATA command after ublk kernel driver returns > cqe with UBLK_IO_RES_NEED_GET_DATA. > > After the kernel side gets the sqe (UBLK_IO_NEED_GET_DATA command), it > now copies(address pinned, indeed) data to be written from bio vectors > to newly returned IO data buffer. > > Finally, the kernel side returns UBLK_IO_RES_OK and ublksrv handles the > IO request as usual.The new feature: UBLK_F_NEED_GET_DATA is enabled on > demand ublksrv still can pre-allocate data buffers with task_work. > > 3. Evaluation: > Related userspace code and tests have been added in ublksrv[1] as one > pull request. We evaluate performance based on this PR. > > We have tested write latency with: > (1) No UBLK_F_NEED_GET_DATA(the old commit) as baseline > (2) UBLK_F_NEED_GET_DATA enabled/disabled > on demo_null and demo_event of newest ublksrv project. > > Config of fio:bs=4k, iodepth=1, numjobs=1, rw=write/randwrite, direct=1, > ioengine=libaio. > > Here is the comparison of lat(usec) in fio: > > demo_null: > write: 28.74(baseline) -- 28.77(disable) -- 57.20(enable) > randwrite: 27.81(baseline) -- 28.51(disable) -- 54.81(enable) > > demo_event: > write: 46.45(baseline) -- 43.31(disable) -- 75.50(enable) > randwrite: 45.39(baseline) -- 43.66(disable) -- 76.02(enable) > > Looks like: > (1) UBLK_F_NEED_GET_DATA does not introduce additional overhead when > comparing baseline and disable. > (2) enabling UBLK_F_NEED_GET_DATA adds about two times more latency > than disabling it. And it is reasonable since the IO goes through > the total ublk IO stack(ubd_drv <--> ublksrv) once again. > (3) demo_null and demo_event are both null targets. And I think this > overhead is not too heavy if real data handling backend is used. > > Without UBLK_IO_NEED_GET_DATA, an additional memcpy(from pre-allocated > ublksrv buffer to user's buffer) is necessary for a WRITE request. > However, UBLK_IO_NEED_GET_DATA does bring addtional syscall > (io_uring_enter). To prove the value of UBLK_IO_NEED_GET_DATA, we test > the single IO latency (usec) of demo_null with: > (1) UBLK_F_NEED_GET_DATA disabled; additional memcpy > (2) UBLK_F_NEED_GET_DATA enabled > > Config of fio:iodepth=1, numjobs=1, rw=randwrite, direct=1, > ioengine=libaio. > > For block size, we choose 4k/64k/128k/256k/512k/1m. Note that with 1m block > size, the original IO request will be split into two blk-mq requests. > > Here is the comparison of lat(usec) in fio: > > 2 memcpy, w/o NEED_GET_DATA 1 memcpy, w/ NEED_GET_DATA > 4k-randwrite: 9.65 10.06 > 64k-randwrite: 15.19 13.38 > 128k-randwrite: 19.47 17.77 > 256k-randwrite: 32.63 25.33 > 512k-randwrite: 90.57 46.08 > 1m-randwrite: 177.06 117.26 > > We find that with bigger block size, cases with one memcpy w/ NEED_GET_DATA > result in lower latency than cases with two memcpy w/o NEED_GET_DATA. > Therefore, we think NEED_GET_DATA is suitable for bigger block size, > such as 512B or 1MB. > > [1] https://github.com/ming1/ubdsrv > > Since V3: > > (1) Touch and clear UBLK_IO_FLAG_NEED_GET_DATA only when > UBLK_F_NEED_GET_DATA is enabled. > > Since V2: > > (1) UBLK_IO_NEED_GET_DATA should support both built-in task_work_add() and > io_uring_cmd_complete_in_task() > > Since V1: > > (1) Add tests to compare (1)2 memcpy, w/o NEED_GET_DATA and (2)1 memcpy, > w/ NEED_GET_DATA to show value of UBLK_IO_NEED_GET_DATA. > (2) rebase on the newest version of ublk_drv > > ZiyangZhang (2): > ublk_cmd.h: add one new ublk command: UBLK_IO_NEED_GET_DATA > ublk_drv: add support for UBLK_IO_NEED_GET_DATA > > drivers/block/ublk_drv.c | 106 ++++++++++++++++++++++++++++++---- > include/uapi/linux/ublk_cmd.h | 18 ++++++ > 2 files changed, 112 insertions(+), 12 deletions(-) >
